Tips for buying MRI

When purchasing an MRI
system, facilities should consider these important factors: the magnet,
gradient system, computer.

As for the MRI magnet, it
should produce a highly homogeneous magnetic field covering as wide a
field of view as possible and provide as much patient space as possible.

The MRI system image
resolution is higher when the gradient system is faster, but than the
field of view is smaller.

Keeping up with the magnet,
so that images are instantly available while the scan progresses, is the
computer system's task in a Magnetic Resonance Imaging system.

Facilities should also
consider the site requirements, which are specific to each institution. In
this area, three issues should be examined: the extent of the magnetic
field, the area occupied by the magnet, and the weight of the magnet.

When selecting the MRI site,
the two most important planning considerations are the fringe field and
the need for a site free from ambient RF electrical noise. A permanent
magnet has a minimal fringe field, but needs careful preconstruction
planning because of its tremendous weight.

To contain the magnetic
fringe field of resistive and superconducting electromagnet systems,
shielding can be used. The fringe field could cause problems at some sites
without shielding.

To contain the fringe field,
two approaches are used for MRI units: active and passive shielding.
Active shielding is a design feature of the magnet, while the passive
shielding involves the use of steel around the magnet.

Generally, the size of the MRI
unit's controlled-access area increases as the operational field strength
increases, for the fringe fields created by superconducting and resistive
magnets. The recommended general-public access limit is 5 G. The distance
to the 5 G line ranges from approximately 9 meters for a 0.5 T magnet to
13 meters for a 1.5 T magnet. For a 3.0 T magnet, the 5 G line is about
one meter further from the isocenter than it is for a 1.5 T magnet, and
therefore larger safety boundaries are required.

With MRI active magnet
shielding, the distance to the 5 G line can be drastically reduced to less
than 4 meters for a shielded 1.5 T magnet and less than 3 meters for a
shielded 0.5 T magnet.

Fringe fields are 3-D; so for
higher field strengths, areas on the floors above and below the imaging
facility may also need controlled access and/or shielding.

Careful site selection is
required for MRI units. The operation of gamma cameras, CRT displays,
electroencephalogram and electrocardiogram monitors, and image intensifiers
- are all affected by the magnetic field; ferromagnetic material in the
surrounding area affects the homogeneity of the static magnetic field.

MRI system technicians can
partially compensate for the effects of large stationary masses on field
uniformity by placing corresponding masses of ferromagnetic material in a
symmetric position in the magnet area. A careful site selection for your
MRI system can eliminate moving ferromagnetic objects such as elevators,
automobiles, or forklifts.

To address the problems
associated with the fringe field, active shielding can be used, as well as
close-fitting steel shields integral to the magnet, or steel shielding in
the walls around the magnet. If MRI system users wish to install large
steel sheets of magnetic shielding, they should keep in mind that these
are expensive, and require innovative construction techniques because of
the weight involved.

Facilities should be sure to
include all these extra precautions in early design considerations before
the purchase of MRI systems, because they raise the cost of construction.
Implementation after installation may be even more costly. Actively
shielded magnets have significantly reduced fringe fields and generally do
not require steel shielding.

External ambient RF signals
can degrade MRI image quality below diagnostically acceptable levels; so
even in the most complex MRI system, the magnet assembly must have some
type of RF shielding. These shield assemblies usually consist of a
complete room of copper or aluminum sheets bonded to a composite plywood
support. Physical and visual access to the room is provided by special
door assemblies and window coverings shielded with copper screening.

MRI specialists should
install, fine-tune, and maintain the equipment since MRI technology is
especially complex and sensitive. Experts should train physicians and
technologists and answer their questions.

Facilities should choose a
supplier whose local MRI service and training resources are extensive and
reliable. The availability of such resources should be guaranteed in
writing within any contract between the supplier and the buyer.

To facilitate future
additions to the network, all newly purchased MRI equipment must be
compatible with DICOM 3.0. DICOM conformance statements should be provided
by the suppliers and should explain in detail what information objects,
service classes, and data encoding are supported by their systems. All
statements should share the same format and vocabulary to facilitate Magnetic
Resonance Imaging system comparisons among suppliers.